The eruption of Vesuvius in AD79 was well known prior to the discovery of Pompeii because of the writings of Pliny the Younger. His uncle, also called Pliny and a famous writer in his own right, was commander of the Roman Navy at Misenum, across the Bay of Naples. Once the eruption was noticed, Pliny the Elder led several ships to the affected area to bring people off. On reaching the area, he was overcome by toxic fumes, and died. Pliny the Younger accompanied him on this voyage, survived it, and wrote several letters about the experience. These have survived as Epistolae VI.16 and VI.20.

Earthquakes are a rare, but not unknown occurrence in Central Italy. The occupants of Pompeii would have been unlikely to associate them with the mountain, which had been quiet since 217 AD. While there there was some speculation that Vesuvius was a volcano (see this treatise by Vitruvius), this was not commonly held. Pliny the Elder's Natural History didn't describe the mountain as possibly volcanic.

There were two parts to the eruption. The first sent ash clouds into the stratosphere, and brought pumice raining down around the region. This was discomforting, but didn't directly threaten the region. During this phase, the majority of the escapes occurred. Pliny the Elder made his trip during this period. The second period destroyed the cities. This didn't send lava creeping down. It threw out clouds of choking gas and rock, which raced down the mountain side. These clouds are called pyroclastic flows, and can reach speeds of up to 450 km/h. Those in Herculaneum didn't have a chance to escape, as they were overcome by the pyroclastic clouds before they could notice the eruption. Those in Pompeii had more of a chance, but it was still a slim one. Volcanic eruptions characterised by major ash production, and pyroclastic flows are called Plinean, after Pliny the Younger.